Understanding the Manganese System

A low temperature NMR experiment of the reaction at different stages may give some insight to the species that are present and their concentrations. This experiment may reveal the stage at which desilylation of the more sterically demanding alcohol is taking place and the acid component that protonates it. We observed a lower yield for the reactions performed with Mn(CO)sBr as the catalyst compared to the reactions performed with 10 % Pd/C as catalyst. The lower yield may be attributed to deactivation of the catalyst by the alcohol as seen by Brookhart et al. with the cationic [Cp(CO)(PR.3)Fe+] catalyst.14 

Y = nucleophilic part of reagent the attacking alkoxide group 

Oxidative addition of the 16-electron manganese center on the silyl ether leads to silyl-manganese complex III. Attack on III by an incoming alcohol (ROH) leads to complex TV. Pseudorotation gives all four possible structures of IV. Complex IV, having a free proton, 

Pathway A is a dissociation of the silyl ketal from the metal center forming a metal-dihydrogen complex V and the unsymmetrical silyl ketal product. Once this product is formed, it cannot get reprotonated because the metal-dihydrogen complex V is not sufficiently acidic to reprotonate silyl ketal. Hydrogen gas then dissociates from the metal center aided by the silane to regenerate the catalyst (III). 

Pathways B and C describe the process that could occur to form the symmetrical silyl ketal. Formation of silyl ketal-metal complex VII could occur in one step via an associated pathway B, or in two steps via a dissociative pathway C. Pathway B could occur by attack on the pentacoordinated silyl ketal-metal complex IV by alcohol ROH, forcing the dissociation R OH leading to complex VII. The associative pathway would precede through a hexacoordinated silicon species. Hexacoordinated silicon compounds have been shown to be very reactive.36,23d The choice of the alcohol that dissociates would be determined by electronic and/or steric factors. 

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Further Developments of Methodology 2.4.1. Understanding the Manganese System. 

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